1. Field of the Invention
The present invention is directed to probe-based instruments, and more particularly, a scanning probe microscope that is operable in a torsional mode by driving the probe into torsional resonance about its longitudinal axis.
2. Description of Related Art
Several probe-based instruments monitor the interaction between a cantilever-based probe and a sample to obtain information concerning one or more characteristics of the sample. Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically use a sharp tip and low forces to characterize the surface of a sample down to atomic dimensions. More particularly, SPMs typically characterize the surfaces of such small-scale sample features by monitoring the interaction between the sample and the tip of the associated probe assembly. By providing relative scanning movement between the tip and the sample, surface characteristic data and other sample-dependent data can be acquired over a particular region of the sample, and a corresponding map of the sample can be generated. Note that “SPM” and the acronyms for the specific types of SPMs, may be used herein to refer to either the microscope apparatus, or the associated technique, e.g., “scanning probe microscopy.”
The atomic force microscope is a very popular type of SPM. The probe of the typical AFM includes a very small cantilever which is fixed to a support at its base and has a sharp probe tip attached to the opposite, free end. The probe tip is brought very near to or into direct or intermittent contact with a surface of the sample to be examined, and the deflection of the cantilever in response to the probe tip's interaction with the sample is measured with an extremely sensitive deflection detector, often an optical lever system such as described in Hansma et al. U.S. Pat. No. RE 34,489, or some other deflection detector such as an arrangement of strain gauges, capacitance sensors, etc.
Preferably, the probe is scanned over a surface using a high-resolution three axis scanner acting on the sample support and/or the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other property of the sample as described, for example, in Hansma et al. U.S. Pat. No. RE 34,489; Elings et al. U.S. Pat. No. 5,226,801; and Elings et al. U.S. Pat. No. 5,412,980.
AFMs may be designed to operate in a variety of modes, including contact mode and oscillating flexural mode. In contact mode operation, the microscope typically scans the tip across the surface of the sample while keeping the force of the tip on the surface of the sample generally constant by maintaining constant deflection of the cantilever. This effect is accomplished by moving either the sample or the probe assembly vertically to the surface of the sample in response to sensed deflection of the cantilever as the probe is scanned horizontally across the surface. In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography. Alternatively, some AFMs can at least selectively operate in an oscillation “flexural mode” of operation in which the cantilever oscillates generally about a fixed end. One popular flexure mode of operation is the so-called TappingMode™ AFM operation (TappingMode™ is a trademark of the present assignee). In a TappingMode™ AFM, the tip is oscillated flexurally at or near a resonant frequency of the cantilever of the probe. When the tip is in intermittent or proximate contact with surfaces the oscillation amplitude will be determined by tip/surface interactions. The amplitude or phase of this oscillation is kept constant during scanning using feedback signals, which are generated in response to tip-sample interaction. As in contact mode, these feedback signals are then collected, stored, and used as data to characterize the sample.
Independent of their mode of operation, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers typically fabricated using photolithographic techniques. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research.
One limiting characteristic of AFMs and other probe-based instruments lies in the above-described modes of operation. In an AFM, the cantilever is typically oscillated using a piezoelectric drive, often known simply as a piezo drive, to provide, for example, a flexural oscillation mode. Referring to FIG. 1, a probe assembly 20 includes a probe 21 having a cantilever 22 and a tip 28. The cantilever 22 extends outwardly from a base 24 of assembly 20. The cantilever 22 may be attached to the base 24 or formed integrally with it. Base 24 is typically coupled to a piezoelectric drive 26 (e.g., a piezo stack). Tip 28 is provided on the opposed, free end of cantilever 22. Piezoelectric drive 26 can be selectively excited by a signal generator 30 to move cantilever 22 up and down relative to a sample 32. When the instrument is configured for flexural oscillation mode operation, the drive voltage is applied to piezoelectric drive 26 to flexurally oscillate the cantilever 22 about a lateral axis of the probe 20 at a frequency that is dependent upon the frequency of the drive voltage.
More particularly, in flexural oscillation mode, cantilever 22 is driven to resonate at its flexural resonance frequency or a harmonic thereof about a lateral axis A–A′ at the base 24 of cantilever 22. Characteristics of cantilever flexural oscillation, and changes thereof, are detected by quadrature photodetector 34, typically with its vertical components, as shown by the arrow “V” in FIG. 1. The deflection angle is sensed by photodetector 34 and output as a voltage signal. Notably, the amplitude of the flexural oscillation ranges between a few nm to 100 nm peak-to-peak depending on the cantilever length.
In operation, as tip 28 approaches a surface of sample 32, the flexural oscillation (tapping) amplitude starts to decrease due to contact between tip 28 and sample 32. Notably, the flexural vibration amplitude decreases to zero when tip 28 is pushed against sample 32 with constant contact pressure. Variation of amplitude between zero (generally continuous contact) and free oscillation is typically used in a feedback configuration to control tip/surface distance. Alternatively, the phase of the flexural oscillation may be used to control this distance. Information relating to the surface such as topology, hardness, and/or electromagnetic properties is then determined by analyzing the signals that are used to control this tip/surface spacing.
Overall, flexural oscillation mode AFMs are used to characterize surface topology and surface energy dissipation by monitoring the amplitude and/or phase of the oscillating cantilever. This mode is often preferred to contact mode imaging because it produces less damage to the tip and sample during operation. However, operating the AFM based on flexural oscillation of the cantilever is constrained in the following aspects.
Initially, flexural mode operation only detects surface characteristics that impart a force in one direction, namely, the vertical or “Z” direction. As a result, flexural mode AFMs do not detect shear force interaction, and thus also cannot provide shear force or force gradient information. This information is critical to making measurements of surface friction, for example, when attempting to identify surface compositional differences. When the topography of the materials is generally undifferentiated, minimal information is provided by flexural mode operation, and thus this friction information becomes particularly valuable, and sometimes necessary. Applications include identifying different components in polymer blends, composites and other mixtures, identifying organic and other contaminants on sample surfaces, delineating coverage by deposited coatings and other surface layers, etc.
Moreover, without shear force or shear force gradient measurement capabilities, flexural mode operation often results in loss of other information relating to the sample. For example, when a flexural oscillation mode AFM is used to image the magnetic domain of a sample, only a force gradient in the direction perpendicular to the sample surface can be sensed. Domains parallel to the surface can only be seen at the domain boundaries where the transitional region has a vertical force gradient. This limitation also holds true for electric force imaging.
Other drawbacks associated with flexural resonance imaging are slow kinetics and small amplitude errors that can drastically limit scanning and data acquisition speed and compromise image integrity. This effect is illustrated in the response curve 40 of FIG. 2. In this case, AO is the free air amplitude of oscillation (in RMS voltage), and AS is the set-point amplitude. When tip/sample separation is reduced, and the tip and sample interact, there is a corresponding change in the signal produced by the deflection detection system. The amplitude of flexural oscillation of the lever decreases due to it being constrained by the sample surface as the tip approaches the surface and taps the sample in each stroke of the oscillation. This is shown in region “O” in which tip-surface distance (x-axis) is smaller than half of the peak-to-peak oscillation of the cantilever. Notably, a feedback loop operates to move the cantilever up and down to keep generally the same oscillation amplitude AS. This movement reflects height changes in the sample, i.e., surface topography.
The response of the cantilever in this flexural mode is illustrated by the slope of the curve at region “O.” In other words, for a particular change in tip/sample separation, the corresponding measured change in voltage is relatively small. It is this measured change that determines the error that is processed by the feedback loop to return operation to the set-point oscillation. Because the slope of the cantilever response in flexural mode is relatively shallow, scan speed must be kept small as relatively large changes in tip-sample separation produce a relatively small change in measured output, or error. Therefore, to facilitate adequate data collection and integration of error signals, the scan time at each location (or image pixel) must be long enough for the system to respond with accuracy and resolution. The speed of data acquisition must be correspondingly limited as well. An improvement in data acquisition speed was desired.
Moreover, the shallow slope of the amplitude/distance curve in FIG. 2 makes the control signal (voltage in the vertical axis) correspond to a large height or distance compared to an amplitude/distance curve with a steeper slope. As a result, the control error will correspond to a greater quantity of height measurement error. The situation is particularly problematic when the probe is scanning across an abrupt step where slower response due to error integration will result in even greater inaccuracy for a given scan speed. Notably, such inaccuracy may be detrimental to obtaining useful data in semiconductor metrology.
Yet another limitation with flexural mode operation is that the flexural resonance is very sensitive to the imaging environment (e.g., when the sample is immersed in water), and thus oscillation properties often change drastically, and in unpredictable ways, upon change in imaging environment. Currently, the sensitivity of flexural mode operation to imaging environment is one of the most significant design considerations when configuring an AFM for operation in fluid.
Other modes of AFM operation are similarly limited. For example, shear force interaction between the probe in contact mode and the corresponding sample surface has been studied with AFM for a number of years. In an AFM technique known as lateral force microscopy (LFM), the cantilever tip is dragged across the sample surface, as in contact mode, to measure friction forces, as described in U.S. Pat. No. 5,553,487 to the present assignee. More particularly, using LFM, the tip is introduced to the sample surface under a constant deflection and then scanned along the surface either in the direction of the cantilever length, or perpendicular to the cantilever length. Using a laser-based deflection detection system, the lateral cells of the corresponding photodetector sense rotation of the cantilever as the tip of the probe interacts with the sample through friction force. In the case where tip scanning direction is perpendicular to the cantilever, the difference of the lateral deflection during forward and reverse scanning of the same portion of the sample is used as a relative measure of the shear force, or surface friction. In addition to the drawbacks associated with using contact mode to detect topology characteristics, including tip/sample damage, etc., LFM suffers the disadvantage of poor lateral resolution and poor repeatability.
In other techniques, the tip placed in contact with the sample surface is modulated by moving the sample surface laterally relative to the probe. In this case, the lateral rocking of the cantilever as a result of the contact friction is used to indicate a quantity of surface friction. However, the lateral deflection signals are small, and thus often unusable, and resolution is insufficient for some of the applications contemplated by the present invention.
In addition, although lateral deflection signals induced by motion of the sample at acoustic frequencies can be enhanced, the main control loop that defines tip/surface relative position still employs vertical deflection (contact mode) feedback and, therefore, suffers the drawbacks associated with that technique, including slow kinetics, and inability to image a shear force gradient.
As a result, the metrology and other research fields were in need of a probe-based instrument capable of detecting multi-directional forces with improved imaging speed. More particularly, an AFM capable of imaging shear forces and shear force gradients with components exhibiting fast response dynamics was desired.